1. Field of the Invention
The present invention relates to a page description data processing apparatus, a page description data processing method, and a recording medium for performing a particular processing sequence on page description data described by a page description language (PDL), thereby to convert the page description data into more robust page description data (hereinafter referred to as “robustized page description data”).
2. Description of the Related Art
In recent years, DTP processes for setting characters and images generated by the operator with a computer into an electronic page using DTP (DeskTop Publishing) application software installed in the computer have been widely used in the printing and platemaking fields.
The DTP application software generates page description data which express images of respective pages based on elements such as characters and images edited by the operator.
The page description data are vector data independent of the resolution of an output machine such as a printer, a platesetter, or the like, and cannot be output per se from the output machine. Therefore, the page description data are rasterized by an RIP (Raster Image Processor) into raster image data comprising a cluster of dots representative of elements such as characters and images in pages.
When the raster image data are supplied to the output machine such as a printer, a platesetter, or the like, the output machine outputs a hard copy or a printing plate carrying an image based on the raster image data (see Japanese Laid-Open Patent Publication No. 2005-070957).
PDF (Portable Document File) version 1.3, which is one type of page description data, incorporates a plurality of processing schemes (hereinafter referred to as “line join styles”) that are applied to joints in stroking a plurality of connected paths. The term “path” refers to a path which has no line width by itself and which interconnects a start point and an end point. The term “stroke” refers to a process of applying a certain line width to a path.
Of the line join styles incorporated in PDF version 1.3, “miter join” is a process of extending the outer edges of two line segments until they meet at a certain angle, as in a picture frame. Another line join style called “bevel join” is a process of squaring off the end points of two line segments and filling a space defined jointly by the square ends with a triangle. For details, reference should be made to PDF Reference, 2nd edition, Adobe Portable Document Format Version 1.3, 1st edition, 1st print, published July 2000, Author: Adobe Systems, Publisher: Pearson Education, ISBN0-201-61588-6, pages 135-136, 140-141.
According to the PDF specifications, it is prescribed that if the angle at which two line segments cross each other is equal to or greater than a certain value (miter limit), then the miter join is applied, and if the angle is smaller than the miter limit, the bevel join is applied. Therefore, if the line segments cross each other at an acute angle, then the corner formed by the crossing line segments is squared off to prevent itself from protruding excessively, with the result that they have an improved image quality, particularly if they produce a character image.
However, even the PDF specifications are susceptible to an unexpected problem which is experienced in a process of rasterizing a line shape that is made up, as shown in FIG. 8A of the accompanying drawings, of a curved path PC having a start point A1 and an end point A2 and a straight path PL having a start point A2 and an end point A3. The problem will be described below while the rasterizing process will be explained.
As shown in FIG. 8B of the accompanying drawings, an object OC (see FIG. 8A) strokes the curved path PC (indicated by the solid line in FIG. 8A), and an RIP approximates the curved path PC with a polygonal path PL1 (indicated by the broken line in FIG. 8B), made up of a plurality of line segments. The approximated result depends on the characteristics inherent in the output machine, such as an output resolution thereof, and the software version of the RIP. In FIG. 8B, the curved path PC comprises a curve A1A2 divided into two line segments by a joint J1.
Then, the shapes of the outer edges of line segments having a certain line width are determined along and over a central line that is represented by the polygonal path PL1. The angle ∠J1A2A3 formed between a line segment J1A2 and a line segment A2A3 varies depending on the position of the joint J1 that has been determined by the above approximation. Consequently, it is impossible to predict which one of the miter join and the bevel join is to be applied as the line join style at the point A2.
If the angle ∠J1A2A3 in FIG. 8B is smaller than the miter limit, then the corner of the point A2 is processed into a bevel join, as shown in FIG. 8C. As a result, a stroked area 4 is produced which is devoid of an apex (cut-off area 2) at the corner of the point A2.
In FIG. 8D of the accompanying drawings, the curve A1A2 is divided into three line segments by joints J2, J3, and is approximated by a polygonal path PL2, indicated by the broken line.
The angle ∠J3A2A3 between a line segment J3A2 and a line segment A2A3 shown in FIG. 8D is greater than the angle ∠J1A2A3 shown in FIG. 8B. If the angle ∠J3A2A3 is equal to or greater than the miter limit, then the corner of the point A2 is processed into a miter join, as shown in FIG. 8E of the accompanying drawings. As a result, a stroked area 6 is produced which has an apex at the corner of the point A2.
As shown in FIGS. 8B and 8D, since the angle at which the curved path PC and straight path PL join each other varies depending on the number of line segments into which the curve A1A2 is divided, the cut-off area 2 (see FIG. 8C) may be irregularly produced. The same phenomenon may occur with respect to a line shape that is made up of joined objects each stroking a curved path.
As described above, when curved paths are approximated by polygonal paths depending on the characteristics inherent in the output machine, such as an output resolution thereof, and the software version of the RIP, since the angle at which the paths join each other is indefinite, they may be converted into an unexpected stroked shape. Accordingly, when image data representing a precipitous line shape are rasterized, the rasterized image data may possibly lead to unexpected print difficulties whose causes are difficult to identify.